In magnetic disk drives such as hard disk drives, a voice coil motor drives an arm supporting a magnetic head to place the magnetic head on a target location of a magnetic disk, thus recording and reproducing information.
To achieve a higher positioning accuracy, there has been recently proposed a magnetic disk drive equipped with so-called dual-stage actuator system, in which a fine actuator, driven by a piezoelectric element or the like, is placed on the arm in addition to a coarse actuator such as a voice coil motor.
One of the control methods for the dual-stage actuator magnetic head positioning mechanism is a method called decoupling-path control disclosed in Patent Document 1. In the decoupling-path control, an amount of displacement of the fine actuator is added to the reference signal of the coarse actuator, and in the case in which direct measurement of the amount of displacement of the fine actuator is not possible or difficult, the amount of displacement is estimated using a fine actuator model.
FIG. 5 shows an example of the dual-stage actuator position control mechanism using the decoupling-path control method. By adding the output of the fine actuator (Micro-Actuator (MA)) PM, which is estimated using the fine actuator model {circumflex over (P)}M and denoted by YM, to an input signal E of a VCM controller GV, the closed loop transfer function of the entire system is defined by a product of the two transfer functions representing individual closed loop systems constructed for actuator PM, and VCM, and if each closed loop system is stable, a stable decoupling-path control system is realized.
In the case where a modeling error exists, as the control system is not completely decoupled, it is required to identify the gain of the fine actuator MA by, for example, the method disclosed in Patent Document 2, thereby calibrating the model gain. Patent Document 2 discloses a dual-stage actuator position control device having a means for generating the command signal to excite the fine actuator at frequency fn, a VCM controller with a notch filter to cut off the frequency component at fn, and an adaptive identification means for estimating the gain of the fine actuator from a command signal of the fine actuator and a head position signal.
[Patent Document 1] JP-A-04-368676
[Patent Document 2] JP-A-2004-30731
It is known that in the fine actuator or the like driven by a PZT element, the gain characteristics vary depending on the amplitude of the input to the fine actuator due to the hysteresis of the PZT element. Therefore, in a method of obtaining the gain by disconnecting the fine actuator from the control loop and directly exciting the fine actuator such as the method disclosed in Patent Document 2, it is required to adjust the amplitude of the excitation signal for gain identification to represent the amplitude of the command signal applied to the fine actuator during the decoupling-path control.
However, since the fine actuator command signal during the decoupling-path control varies due to the gain of the fine actuator, due to characteristics of the coarse actuator, due to disturbances acting on the control system, and other factors, it is desirable to measure the actual fine actuator command signal during the decoupling-path control for each head and each drive, and adjust the excitation signal accordingly for gain identification. However, since the fine actuator command signal during the decoupling-path control also varies due to the gain of the fine actuator model which is yet to be calibrated, it is difficult to accurately determine the fine actuator command signal in general, and as a result, there always exists an error in the calibrated fine actuator model gain which causes degradation in the performance during the decoupling-path control.
The present invention has been made in view of the above problem to provide a magnetic disk drive for calibrating the gain of the fine actuator model based on the behavior of the fine actuator during the decoupling-path control.